This invention relates to a high frequency (micro-wave) dielectric filter which uses a dielectric resonator.
The basic structure of a dielectric filter for use a TM (Transverse Magnetic) mode which is comprised of a metal case and a dielectric resonator is disclosed in a scholarly paper (Kobayashi et al.: IEEE MTT-S Digest, p 233-235 (1978)). In such a basic structure, however, the relative positional relationship between the dielectric resonator and the case is changed due to vibration and impact and hence, the characteristics of the dielectric filter is changed. Accordingly, such a dielectric filter is not practical.
Furthermore, in case the case and the dielectric resonator are fixed to each other by some methods, there is a chance that the quality factor is remarkably deteriorated and accordingly the dielectric filter is not usable in such a case.
In view of the above, conventionally, a method in which the case and the dielectric resonator are fixed to each other by means of a conductive paste or adhesive agent has been adopted.
The frequency band ranging from a few hundreds MHz to a few GHz has been widely used in various fields, chiefly in a mobile radio communication system. Here, the dielectric filter which works as a passive element is required to sustain little loss in quality and to be stable against the external environment.
The dielectric filter using the TM mode resonance is constructed such that a pillar-like dielectric has both ends thereof securely fixed to a metal case. It is indispensable that the dielectric filter has a relative positional relationship which does not change between the dielectric resonator and the case, and has highly stable against vibration and impact. From this point of view, there has been a practical problem that the conventional conductive paste or adhesive agent is less than optimal in terms of the stability, deterioration of the quality factor being likely to occur.
As described above, the fixing method which uses conductive paste or the adhering agent easily gives rise to dispersion by irregularities of the amount of coating of the dielectric filter. Therefore, the successful application of the fixing method largely depends on the experience of the workman and it is difficult to manufacture products of uniform characteristics. Furthermore, because of the poor reproducibility, it is not necessarily suitable the mass production.